One of the process steps commonly encountered in the fabrication of integrated circuits and other semiconductor devices is photolithography. Broadly, photolithography involves selectively exposing a specially prepared wafer surface to a source of radiation using a patterned template to create an etched surface layer. Typically, the patterned template is a reticle, which is a very flat glass plate that contains the patterns to be reproduced on the wafer. For example, the wafer surface may be prepared by first depositing silicon nitride on it followed by a coating of a light-sensitive liquid polymer or photoresist. Next, ultraviolet (UV) light is shone through or reflected off a surface of a mask or reticle to project the desired pattern onto the photoresist-covered wafer. The portion of the photoresist exposed to the light is chemically modified and remains unaffected when the wafer is subsequently subjected to a chemical media that removes the unexposed photoresist leaving the modified photoresist on the wafer in the exact shape of the pattern on the mask. The wafer is then subjected to an etch process that removes the exposed portion of the nitride layer leaving a nitride pattern on the wafer in the exact design of the mask. This etched layer, singly or in combination with other similarly created layers, represent the devices and interconnects between devices characterizing the “circuitry” of a particular integrated circuit or semiconductor chip.
The industry trend is towards the production of chips that are smaller and/or with a higher logic density necessitating even smaller line widths on larger wafers. Clearly, the degree of fineness to which the surface of the reticle can be patterned and the degree to which this pattern can be faithfully replicated onto the wafer surface are factors that impact the quality of the ultimate semiconductor product. The resolution with which the pattern can be reproduced on the wafer surface depends on the wavelength of ultraviolet light used to project the pattern onto the surface of the photoresist-coated wafer. State-of-the-art photolithography tools use deep ultraviolet light with wavelengths of 193 nm, which allow minimum feature sizes on the order of 100 nm. Tools currently being developed use 157 nm Extreme Ultraviolet (EUV) light to permit resolution of features at sizes below 70 nm. The reticle is a very flat glass plate that contains the patterns to be reproduced on the wafer.
Typical reticle substrate material is optically clear quartz. Because of the tiny size of the critical elements of modern integrated circuits, it is essential that the operative surface of the reticle (i.e. the patterned surface) be kept free of contaminants that could either damage the surface or distort the image projected onto the photoresist layer during processing leading to a final product of unacceptable quality. Typically, the critical particle sizes are 0.1 .mu.m and 0.03 .mu.m for the non-patterned and patterned surfaces respectively when EUV is part of the photolithography process.
Typically, the patterned surface of the reticle is coated with a thin, optically transparent film, typically of nitrocellulose, attached to and supported by a frame, and attached to the reticle. Its purpose is to seal out contaminants and reduce printed defects potentially caused by such contamination in the image plane. With EUV, however, reflection from the patterned surface is used as opposed to transmission through the reticle characteristic of deep ultraviolet light photolithography. At his time, the art does not provide pellicle materials that are transparent to EUV. Consequently, the reflective photomask (reticle) employed in EUV photolithography is susceptible to contamination and damage to a far greater degree than reticles used in conventional photolithography. This situation imposes heightened functional requirements on a container or pod designed to store, transport and ship a reticle destined for EUV photolithography use. Generally, reticles are stored and/or transported within a mini-clean room type environment created within a SMIF container or pod. Such a container typically includes a door and a cover that mates with the door to form a hermetically sealed enclosure for holding the reticle. The door is generally designed and equipped with special features and mechanisms to enable interfacing with a process tool for automatic or manual opening of the door and subsequent transfer of the reticle to the process tool environment without exposing the reticle to the ambient atmosphere.
Considering the severe impact of particulates on semiconductor fabrication, unnecessary and unintended contact between the reticle and other surfaces during manufacturing, processing, shipping, handling, transport or storage is highly undesirable in view of the susceptibility of the reticle to damage to the delicate features on the patterned surface due to sliding friction and abrasion. Secondly, any particulate contamination of the surface of the reticle could compromise the reticle to a degree sufficient to seriously affect any end product obtained from the use of such a reticle during processing. Particles can be generated within the controlled environment containing the reticle during processing, transport and shipping. Sliding friction between the reticle and the container and consequent abrasion is one of the sources of contaminating particulates. Such a situation can arise while trying to position the reticle inside the container or due to relative movement between the reticle and the container during transport or shipping. For example, a reticle can slide from its position within a reticle container during transport thereby generating particulates. Deformation of the walls of the container can be sufficient to introduce a shift in the position of the reticle within the container. Such a mispositioned reticle will also likely be misaligned when automatically retrieved from the container and positioned into processing equipment leading to an end product of unpredictable quality. Shock and vibration of the container can be transmitted to the reticle and components holding the reticle causing relative movement and associated particle generation. There is also the possibility that the reticle or pellicle might be scratched or crack under such conditions. Of course, the source of particulates can be airborne particulates settling on the reticle. Typically, this problem is mitigated by utilizing hermetically sealed SMIF containers to create and maintain a controlled environment around the reticle.
This discussion is equally applicable to containers designed to transport and/or store semiconductor wafer substrates and reticles that are destined for non-EUV related semiconductor fabrication. For example FOUPS (acronym for front opening unified pod) and FOSBS (acronym for front opening shipping box), and SMIF (acronym for sealed mechanical interface).
Recognizing the need for a controlled environment around the wafer, particularly during storage, processing and transport, prior art has evolved approaches to securely hold a reticle in a fixed position within the reticle container during operations involved in the storage, shipment and transport of the reticle. The most common approach involves providing supports, on a bottom surface or door of the pod, that contact the reticle patterned surface and hold it in a substantially planar configuration with respect to a surface of the container. Very often, the holding supports are augmented with one or more pressing members, extending from the cover or shell of the pod, that contact the reticle on a surface opposite the patterned surface. While this arrangement may serve to restrain movement of the reticle perpendicular to the patterned surface, it is ineffective to hold the reticle against translational movement in the plane of the patterned surface. In this regard, the prior art discloses limiting structures disposed along the periphery of the reticle all being effective to limit the lateral motion of the reticle. The prior art attempts to securely hold the reticle in the container also extend to providing a latch in combination with all of the above structural members. The latch is designed to hold the cover firmly pressed shut against the door or base thereby causing the pressing members to firmly bear down against the reticle. The pressing members may be made of resilient material or mounted at the end of cantilevered arms extending from the cover so that the pressing members can make contact and press against the reticle surface progressively as the cover is brought into engagement with the door. The cantilevered arrangement is purported to allow the application of a compliant and controlled force to the reticle by the reticle supports. Such a compliant and controlled force is said to firmly secure the reticle within the container without excessive forces on or deformation of the reticle, even under circumstances where the container may be slightly deformed. It will become readily apparent to one of ordinary skill in the art that these structures will not prevent relative sliding between the reticle and the support members, the limiting structures and the pressing members. This is particularly true where the container is likely to be subjected to shock and vibration loading. Sliding causes abrasion of the reticle surface and generates particulates.
In recognition of this problem, prior art containers include posts, mounted to the door of the container, for supporting the four respective corners of a reticle. Each corner of the post includes beveled concavities having sloped surfaces at right angles to each other. When a reticle is lowered into the reticle supports, there will be a single horizontal plane where the edge of the reticle lies in contact with each sloped surface of the beveled concavities. The reticle will quickly, easily and repeatably locate in this “single solution” position as a result of the weight of the reticle and low friction between the reticle edges and surfaces of the beveled concavities. The sloped surfaces of each beveled concavity is brought into engagement with a chamfer around a lower edge of the reticle so that the reticle is securely supported at its four corners without the reticle support coming into contact with an upper or lower surface of the reticle, or vertical edges of the reticle. The pressing members may include beveled concavities inverted with respect to the beveled concavities on the reticle supports so that once a reticle is located in the reticle supports, coupling the container cover with the container door will cause the sloped surfaces of each beveled concavity to engage a chamfer around an upper edge of the reticle so that the reticle is sandwiched between the reticle support and pressing members at its four corners so that the reticle is held securely in position during transport of the container and/or a shock to the container.
Some SMIF containers include posts, mounted to the door or base of the container, for supporting the four respective corners of a reticle. Each corner of the post may include beveled concavities having sloped surfaces at right angles to each other. When a reticle is lowered into the reticle supports, there will be a single horizontal plane where the edge of the reticle lies in contact with each sloped surface of the beveled concavities. The reticle will quickly, easily and repeatably locate in this “single solution” position as a result of the weight of the reticle and low friction between the reticle edges and surfaces of the beveled concavities. The sloped surfaces of each beveled concavity is brought into engagement with a chamfer around a lower edge of the reticle so that the reticle is securely supported at its four corners without the reticle support coming into contact with an upper or lower surface of the reticle, or vertical edges of the reticle. The pressing members may include beveled concavities inverted with respect to the beveled concavities on the reticle supports so that once a reticle is located in the reticle supports, coupling the container cover with the container door will cause the sloped surfaces of each beveled concavity to engage a chamfer around an upper edge of the reticle so that the reticle is sandwiched between the reticle support and pressing members at its four corners so that the reticle is held securely in position during transport of the container and/or a shock to the container.
The SMIF containers of the prior art do not minimize contact with the reticle as a whole. In effect, the support arrangements permit substantial sliding contact between the reticle support structures and the reticle before the reticle is brought into position within the container. All such contact may generate particulates and/or affect the pattern etched in the reticle. Additionally, prior art attempts to securely support the reticle in a fixed position within the container introduce additional contacts with the reticle that are likely to cause additional scraping and abrasion of the reticle as it is brought into and out of engagement with the restraints as the reticle is placed in and removed from the container.
The problem of particle generation within the microenvironment is exacerbated when the container is used to ship the reticle. Such a container will encounter diverse operational conditions. One of the operational hazards is that the container will be subjected to shock and vibration loading tending to dislodge the reticle from its secured position within the container. The container could also deform under the impact thereby causing the internal structures attached to the reticle to move and thereby causing the reticle to be misaligned within the container, hi this regard, isolation of the container from shock, as opposed to isolation of the reticle from the container, is an important consideration.
Particle settling is another problem to be considered. It is desirable that particulates that are generated or are otherwise introduced within the controlled environment cannot easily settle on the reticle. In this regard, it is preferable not only to have a minimal volume for the environment within which the reticle is carried and which has to be controlled to avoid particulate contamination but it is also desirable that the air in the controlled volume remain relatively static. Sudden pressure changes or large pressure changes can cause a sudden evacuation or injection of air into the controlled volume leading to turbulence. A filter surface or a wall of the container deflecting in response to large and sudden pressure differences can cause a pressure wave inside the controlled volume leading to particulate migration.
Another challenge to be overcome is the fact that even with a controlled environment, migration of particulates that may be present inside the controlled environment is still possible due to pressure changes of the air trapped in the controlled environment or turbulence of the trapped air brought on by rapid movements of the container or by disturbing the trapped air volume. For example, thin walled SMIF pods may experience wall movement due to altitude related pressure changes causing the trapped air inside the controlled environment to be displaced. Temperature changes can set up convection currents within the container. Dimensional changes of the container and its components can compromise the functioning of support and retaining mechanisms leading to wafer misalignment or warping of the substrate carried within the container. Dimensional changes of the container wall due to pressure fluctuations can lead to compromising the sealing between cover and door of the carrier and incursion of particulates within the carrier. Prior art approaches contemplate a breathing apparatus between the external environment and the internal controlled volume of air. The breathing apparatus provides a path for the air to flow. A filter interposed in the path is expected to provide a barrier to incursion of particulates from the external environment into the controlled environment of the carrier. However, as noted above, the reticle used in a EUV photolithography process has very fine and delicate features so the critical particle sizes are only of the order of 0.1 [mu]m and 0.03 [mu]m for the non-patterned and patterned surfaces of the reticle respectively. At such low particle sizes, a filter would require a very fine pore size causing a considerable resistance to fluid flow across it thereby necessitating a larger filter surface area. The alternative to a larger filter surface area is a slower response to sudden pressure changes such as those encountered in shipping the container. Both of these are not preferred alternatives because one of the objectives of reticle SMIF pod design is to keep the controlled volume to a minimal so it can be effectively sealed against incursion of particulates. Minimizing the controlled volume within which the reticle is positioned whilst providing for a large filter area to achieve pressure equalization within the controlled volume are inconsistent objectives.
Typically, prior art controlled environment is created by interposing a seal between the door and cover. However, very often the seal is made of an elastomeric material which, can be in and of itself a source of particulates or contamination. Moreover, the prior art attempts to create a seal using elastomeric seals requires structures, such as grooves and raised tabs for example, which may provide a path for the particulates to enter the inner controlled environment.
Notwithstanding their widespread use, it is generally accepted in the art that such structures present interstices which are not easy to clean when cleaning the pod thereby potentially retaining chemicals and particulates from the runoff cleaning solution.
What is needed is a reticle containment system that provides maximum protection for the reticle from particles and contamination by providing stable and secure support and a controlled environment. This should include a reticle pressure equalization system that effectively equalizes pressure between an internal controlled environment of the carrier and the air external to the carrier without incursion or excursion of air from the controlled environment and with minimal turbulence of the air already present within the controlled environment. What is also needed is a sealing system that does not utilize any form of a particulate generating material.